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SubjectRe: [BUG] sched: fair group's bug
on 2009-1-8 9:30 Miao Xie wrote:
> I tested fair group scheduler on my hyper-threading x86_64 box(2 CPU * 2 HT)
> and found the deviation of the groups' CPU usage was larger than 2.6.26
> when *offline* a CPU or do hotplug frequently. It is less than 1% On 2.6.26,but
> On current kernel, it is often greater than 4%, even than 10% by accident.
>
> A test program which reproduces the problem on current kernel is attached.
> This program forks a lot of child tasks and attachs these child tasks into each
> CPU controller group, then the parent task gets and checks the CPU usage of
> every group every 5 seconds.(All of the child tasks do the same work - repeat
> doing sqrt)
> Child Task 1
> while(!end)
> sqrt(f);
> Group 1 ......
> Child Task m
> while(!end)
> sqrt(f);
> -----------------------------
> Parent Task Group 2 ......
> get and check the CPU ...... ......
> usage of every group -----------------------------
> Child Task m*n-m+1
> while(!end)
> sqrt(f);
> Group n ......
> Child Task m*n
> while(!end)
> sqrt(f);
>
> Steps to reproduce:
> # mkdir /dev/cpuctl
> # mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
> # ./cpuctl

Sorry. I forgot the step of offline CPU. The correct steps is following:
# echo 0 > /sys/devices/system/cpu/cpu3/online
# mkdir /dev/cpuctl
# mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
# ./cpuctl

> output on current kernel:
> -------------------------
> Group Shares Actual(%) Expect(%)
> 0 1024 41.68 50.00
> 1 1024 58.32 50.00
> 0th group's usage is out of range(deviation = 8.32)
> 1th group's usage is out of range(deviation = 8.32)
> --------------------------
>
> output on 2.6.26
> -------------------------
> Group Shares Actual(%) Expect(%)
> 0 1024 50.03 50.00
> 1 1024 49.97 50.00
> --------------------------
>
> On 2.6.26, the deviation may be greater than 4% at the beginning, but the scheduler
> adjusts soon, and don't occur the large deviation for ever.
>
> Bisect located below patch:
> commit c09595f63bb1909c5dc4dca288f4fe818561b5f3
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri Jun 27 13:41:14 2008 +0200
>
> sched: revert revert of: fair-group: SMP-nice for group scheduling
>
> Try again..
>
> Initial commit: 18d95a2832c1392a2d63227a7a6d433cb9f2037e
> Revert: 6363ca57c76b7b83639ca8c83fc285fa26a7880e
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
>
> Besides that, We found other problems by the attached program.
> 1. some tasks are hungry in the fair group.
> Steps to reproduce:
> # mkdir /dev/cpuctl
> # mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
> # ./cpuctl -g 1 -v
> --------------------
> 1th Check Result:
> Group Shares Actual(%) Expect(%)
> 0 1024 100.00 100.00
> Each task's usage:
> Task in Group 0:
> Task Usage(%)
> 5395 0.000000
> 5396 0.000000
> 5397 0.000000
> 5398 16.677785
> 5399 16.677785
> 5400 16.744496
> 5401 16.611074
> 5402 33.288859
>
> 2. some groups broke the limit of the fair group and get more CPU time When
> the groups is hiberarchy. Such as:
> top group
> |
> group 1
> / \
> task1 group 2
> |
> task 2
> Steps to reproduce:
> # mkdir /dev/cpuctl
> # mount -t cgroup -o cpu,noprefix xxx /dev/cpuctl
> # ./cpuctl -H
> -------------------------
> Group Shares Actual(%) Expect(%)
> 0 1024 60.17 88.89
> 1 1024 39.83 11.11
>


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